The fixed bed catalytic reactor is a well-known, elegant device for carrying out a chemical reaction utilizing a catalyst. There are myriad advantages associated with this type of reactor, such as: the apparatus is typically simple to design, there are no moving parts to wear out, the catalyst stays in the reactor, it is easy to separate the reaction mixture from the catalyst, heat can be added or removed by, for example, the addition of cold gas or liquid quench, internal or external heat exchanger(s), wall heat transfer (e.g., in the case of small diameter tubes like bench scale units or multi-tube-bundle reactors), or the reactor can be operated adiabatically.
There are numerous configurations of fixed bed catalytic reactors, the most common of which is probably cocurrent gas-liquid downflow, described, for instance, by R. Gupta, in “Cocurrent Gas-Liquid Downflow in Packed Beds”, Chapter 19, of the Handbook of Fluids in Motion (1983). Other configurations include cocurrent upflow and countercurrent operations.
Whatever the specific configuration, theoretically the fixed bed catalytic reactor is expected to provide, among other attributes, sufficient volume and residence time to provide the desired conversion, provide sufficient mass transfer rate of reactants and products through the gas-liquid interface and through the liquid film on the surface of catalyst particles, provide effective use of the entire catalyst particle and active sites throughout the cross section of particles in the bed, provide uniform flow distribution over entire width and length of bed to utilize all of the catalyst, provide conditions where gas and liquid phases remain homogeneously mixed and do not separate, allow for conditions where all the catalyst is adequately wetted such that both reactants are present and heat is transferred effectively from all zones in the reactor, provide an effective method for controlling temperature in a safe operating window or effective range to maximize reaction selectivity, product quality, catalyst life, and the like. See for instance H. Hofmann: “Multiphase Catalytic Packed Bed Reactors”, Catal. Rev. Sci. Eng. 17 (1978) 71-117. However, it is still a long-sought goal to achieve all of the aforementioned attributes in a commercial reactor.
An example of the type of process that can be carried out in such a reactor is hydrogenation. Heterogeneous catalytic hydrogenation processes of various kinds are widely practiced on a commercial scale and are used for hydrogenation of a wide variety of organic feedstocks.
Specific examples include hydrogenation of aldehydes and ketones to alcohols, of unsaturated hydrocarbons to saturated hydrocarbons, of acetylene-derived chemicals to saturated materials, of unsaturated fatty acids to saturated fatty acids, of esters of unsaturated fatty acids to esters of partially or fully hydrogenated fatty acids, of nitrites to primary amines, of certain sugars to polyhydroxyalcohols. Other examples include the hydrogenation of quinones (for example the hydrogenation of 2-ethylanthraquinone as a step in the production of hydrogen peroxide), the production of cyclohexanol from cyclohexanone, the production of iso-propanol from acetone, and the hydrogenation of unsaturated hydrocarbons such as in the production of cyclohexane from benzene.
Typical catalysts for such hydrogenation reactions include Group VIII metal catalysts such as cobalt, nickel, rhodium, palladium and platinum (using the traditional CAS version of the Periodic Table; see Chemical and Engineering News, 63(5) 27, 1985), and also other metals such as copper, zinc, and molybdenum.
Production of butane-1,4-diol by hydrogenation of but-2-yn-1,4-diol is an example of hydrogenation of an acetylene-derived chemical; a suitable catalyst for this reaction has been described as a granular nickel-copper-manganese on silica gel. The production of stearic acid by catalytic hydrogenation of the corresponding unsaturated acids, linoleic acid and linolenic acid, using a nickel, cobalt, platinum, palladium, chromium or copper/zinc catalyst, is an example of the hydrogenation of unsaturated fatty acids to yield saturated fatty acids. So-called “hardening” of vegetable oils is an example of hydrogenation of esters of unsaturated fatty acids. Production of beta-phenylethylamine by hydrogenation of benzyl cyanide is an example of hydrogenation of a nitrile. As examples of hydrogenation of sugars to polyhydroxyalcohols there can be mentioned hydrogenation of ketose and aldose sugars to hexahydroxyalcohols, for example hydrogenation of D-glucose to sorbitol and of D-mannose to mannitol.
An important route to C3 and higher alcohols involves hydroformylation of olefins, such as ethylene, propylene, and butene-1, to yield the corresponding aldehyde having one more carbon atom than the starting olefin, followed by hydrogenation to the alcohol. The commercially important Oxo Process comprises such a hydroformylation process, followed by hydrogenation. Thus, hydroformylation of ethylene yields propionaldehyde and propylene yields a mixture of n- and iso-butyraldehyde, followed by catalytic hydrogenation to the corresponding alcohols, e.g. n-propanol and n-butanol. The important plasticiser alcohol 2-ethylhexanol may be made, for instance, by alkali-catalyzed condensation of n-butyraldehyde to yield the unsaturated aldehyde, 2-ethyl-hex-2-enal, which is then hydrogenated to yield the desired 2-ethylhexanol. Historically the preferred catalysts for such aldehyde hydrogenation reactions are the Group VIII metal catalysts, particularly nickel, palladium, platinum, or rhodium. Numerous other systems have been proposed. The Oxo Process and variations thereon are the subject of numerous patents and patent applications, more recent examples of which are WO2003083788A2 and WO2003082789A2, and which in turn recite numerous references to the same subject matter.
Hydrodesulphurisation is another commercially important hydrogenation reaction. This is the removal of complex organic sulfur compounds, such as sulfides, disulfides, benzothiophene and the like, from a mixed hydrocarbon feedstock by catalytic reaction with hydrogen to form hydrogen sulfide.
Similar, and often simultaneously to hydrodesulfurization is hydrodenitrogenation, where complex organic nitrogen components are converted with hydrogen to form hydrocarbons and ammonia. Typical organic nitrogen components are pyrrole, pyridine, amines and benzonitriles.
Another refining application is hydrocracking which is used to reduce the boiling point of the feed by cracking large molecules into smaller ones and adding hydrogen to them using a bifunctional catalyst.
Catalytic hydrotreating is, in all the above cases, a heterogeneous process, typically operated as a vapour phase process or as a liquid/gas phase process. In the conventional multi-stage hydrogenation processes the hydrogen-containing gas and the material to be hydrogenated are fed through the plant in co-current or in counter-current fashion. In order to achieve good economy of hydrogen usage, sometimes a recycle gas is used, typically comprising H2 and a diluent such as methane other light product gases of the main process.
The term “trickle bed reactor” or “trickle bed state” is often used to describe a reactor in which a liquid phase and a gas phase flow cocurrently downward through a fixed bed of catalyst particles while reaction takes place. However, these reactors can be operated in various flow regimes, depending on vapor and liquid flow rates and properties. At sufficiently low liquid and gas flow rates the liquid trickles over the packing in essentially a laminar film or in rivulets, and the gas flows continuously through the voids in the bed. This is termed the gas continuous region or more specific “trickle flow regime” and is the type encountered usually in refinery applications, in which typically large excess of hydrogen is required to prevent coking and to keep the concentration of catalyst poison such as hydrogen sulfide that is formed during the reaction low. It should be noted, however, that the operating window of trickle flow is very wide and not only determined by flow rates (see, e.g., E. Talmor, AIChE Journal, Vol. 23, No. 6, 868-874 (November 1977) discussed more fully below). Thus, for instance, and without wishing to be bound by theory, it may be possible to operate with low liquid rates but at relatively high gas rates, too.
As gas and/or liquid flow rates are increased there is encountered behavior described as rippling, slugging, known in the art as “pulse flow regime”. Such behavior may be characteristic of the higher operating rates often encountered in commercial petroleum processing. Pulsing is caused by alternating zones that are rich in vapor or in liquid. It is often called “high interaction flow regime”. At high liquid rates and sufficiently low gas rates, the liquid phase becomes continuous and the gas passes in the form of bubbles; this is termed “dispersed bubble flow” or “bubble flow regime” and is characteristic of some chemical processing in which liquid flow rates (per unit cross section area of the reactor) may be comparable to but more typically are much higher than the highest encountered in petroleum processing, but where gas/liquid ratios are much less.
Fixed bed hydrogenation reactors running in the bubble flow regime, i.e., relatively low volumetric gas to liquid ratio can have a tendency for the phases to separate. Without wishing to be bound by theory, such phase separation may happen because the separate flows have a pressure drop over the reactor or part of the reactor that is lower than the mixed phase pressure drop would be; notionally, this may result in loss of much of the reaction due to starvation of one component in those vapor-liquid separated zones, “hot-spots” resulting from the localized accumulation of reaction heat that is not transported away by an adequate flow of liquid, and/or overall axial or centerline temperature profile showing a higher ΔT than what is theoretically possible under ideal flow conditions.
The aforementioned problem is avoided in most refinery processes where a large excess of gas is recycled over the reactor, putting it into the trickle flow or pulsing regime. Another way to solve the problem is to use high liquid velocities, with resulting high pressure drop along with high energy consumption. Both alternatives are high cost solutions.
What is needed is a simple and direct way of optimizing reactor operation and/or design to combine the key variables such as pressure, feed flows, particle size, bed void fraction, and reactor dimensions, and to do so economically. Unfortunately, the literature appears to be consistent in describing the phenomena of multiphase flow in a packed bed as complex and not well understood. Once the process chemistry, catalyst, and temperature are defined, the engineer is faced with the selection of a number of parameters which will determine the overall effectiveness of the process. Variables include reactor diameter, reactor length, pressure, liquid rate, gas rate, gas/liquid stoichiometry, catalyst particle size, catalyst particle shape, catalyst loading density, number of vessels, heat removal method, among others.
Undaunted by the complexity of the system, there have been numerous attempts in the past to optimize reactor operation and/or design using selected variables. Some limited successes have been claimed for certain types of chemical reactions and/or using certain flow regimes.
U.S. Pat. No. 4,288,640 describes cocurrently passing a gas and liquid through a packed column in the form of a turbulent stream at rates such that, when the gas flow rate is kept constant, a variation in flow rate of the liquid produces a rise in the pressure difference Δp with increasing liquid flow load L, expressed as Δp/L, at least twice as large as the rise Δp/L under liquid trickling conditions providing laminar flow of the liquid over the packing bodies and continuous gas flow but the liquid flow rate being below the rate at which pulsing Δp in the column is produced, using specified packing.
U.S. Pat. No. 5,081,321 describes a catalytic hydrogenation process to produce isopropanol by feeding hydrogen gas and acetone into a fixed bed reactor forming a cocurrent gas-liquid downflow while maintaining the catalyst bed in a trickle bed state, wherein the following equation is met: {B/[A·(σ/100)]}>1, wherein B is moles of hydrogen, A is moles of acetone, and σ is percent conversion of acetone, provided that a trickle flow state is maintained.
U.S. Pat. No. 5,093,535 teaches a cocurrent hydrogenation process having a hydrogenation zone containing a bed of catalyst whose particles lie in the range of 0.5 mm to 5 mm and maintaining the supply of feedstock to the bed so as to maintain a superficial liquid velocity of liquid down the bed in the range of from about 1.5 cm/sec to about 5 cm/sec while controlling the rate of supply of the H2-containing gas to the bed so as to maintain at the top surface of the bed of catalyst particles a flow of H2-containing gas containing from 1.00 to about 1.15 times the stoichiometric quantity of H2 theoretically necessary to convert the organic feedstock to the hydrogenation product.
FR 2,597,113 relates to trickle phase method for the selective hydrogenation of highly unsaturated hydrocarbons, the method characterized in that the product used, which contains the highly unsaturated components and the hydrogenation gas, which contains hydrogen, are directed through the catalyst at a surface flow velocity with respect to the geometric surface of the particles of the total quantity of catalyst of 1.5×10−7 to 3.0×10−5 m/s relative to the hydrogenation phase.
The prior art methods, however, suffer inter alia by making use of only a very small number of reactor variables available and thus are too restrictive, and/or do not provide results on pilot plant or laboratory scale operations that can consistently be scaled up to commercial scale reactors.
E. Talmor, AIChE Journal, Vol. 23, No. 6, p. 868-874 (November 1977) teaches one type of flow map to describe the flow regimes in downflow through packed beds and found that the flow regimes encountered in such systems depend on the superficial volumetric gas-to-liquid ratio and the ratio of inertia plus gravity forces to viscous plus interphase forces.
Flow maps attempt to predict the flow regime at a given set of measurable conditions, but studies of this sort do not usually make predictions or recommendations as to how well a chemical reaction would occur in the predicted flow regime.
Flow maps have been used in a number of patents to ascertain useful hydraulic regimes (e.g., trickle, pulse, or bubble), e.g., U.S. Pat. No. 6,774,275 (WO2004016714) and the aforementioned U.S. Pat. No. 5,081,321. Numerous other publications concern this subject matter, e.g., Sherman et al., “Kinetic and Hydrodynamic Effects in the Activity Testing of Hydrodesulfurization Catalyst in Packed-Bed Reactors”, Symposium, Modeling and Troubleshooting of Commercial-Scale Reaction Systems, AIChE 71st Annual Meeting, Nov. 12-16, 1978; Morsi et al., “Flow Patterns and Some Holdup Experimental Data in Trickle-Bed Reactors for Foaming, Nonfoaming, and viscous Organic Liquids”, AIChE Journal, Vol. 24, No. 2, pp. 357-360, March 1978. Talmor-like maps do not per se concern optimal conditions for a fixed bed reactor; they attempt to predict the flow regime. Typically flow maps are designed using water and air and fail when applied to other fluid systems.
Among other references discussing hydraulic conditions or related factors for operating a reactor include: U.S. Pat. Nos. 4,851,107; 6,492,564; and 6,680,414; Holub et al., “Pressure Drop, Liquid Holdup, and Flow Regime Transition in Trickle Flow, AIChE Journal, Vol. 39, No. 2, pp. 302-321, February 1993; Tosun, “A Study of Cocurrent Downflow of Nonfoaming Gas-Liquid Systems in a Packed Bed. 1. Flow Regimes: Search for a Generalized Flow Map”, Ind. Eng. Chem. Process Des. Dev. 1984, 23, 29-35; Cheng et al., “Influence of hydrodynamic parameters on performance of a multiphase fixed-bed reactor under phase transition”, Chemical Engineering Science 57 (2002) 3407-3413; Stuber et al., “Partial Hydrogenation in an Upflow Fixed-Bed Reactor: A multistage Operation for Experimental Optimization of Selectivity”, Ind. Eng. Chem. Res. 2003, 42, 6-13; Al-Dahhan et al., “High Pressure Trickle-Bed Reactors: A Review, Ind. Eng. Chem. Res. 1997, 36, 3292-3314; Moreira et al., “Influence of Gas and Liquid Flow Rates and the Size and Shape of particles on the Regime Flow Maps Obtained in Cocurrent Gas-Liquid Downflow and Upflow through Packed Beds”, Ind. Eng. Chem. Res. 2003, 42, 929-936; Attou et al., “A two-fluid hydrodynamic model for the transition between trickle and pulse flow in a cocurrent gas-liquid packed-bed reactor”, Chemical Engineering Science 55 (2000) 491-511; Herskowitz et al., “Effectiveness Factors and Mass Transfer in Trickle-Bed Reactors”, AIChE Journal, Vol. 25, No. 2, pp. 272-283; Gianetto et al., “Hydrodynamics and Solid-Liquid Contacting Effectiveness in Trickle-Bed Reactors”, AIChE Journal, Vol. 24, No. 6, pp. 1087-1104; Worstell et al., “Properly Size Fixed-Bed Catalytic Reactors”, Chemical Engineering Progress, June 1993, pp. 31-37; Dudukovic, “Catalyst Effectiveness Factor and Contacting Efficiency in Trickle-Bed Reactors”, AIChE Journal, Vol. 23, No. 6, pp. 940-944; Morita et al., “Mass Transfer and Contacting Efficiency in a Trickle-Bed Reactor”, Ind. Eng. Chem, Fundam., Vol. 17, No. 2, 1978; Ng et al., “Trickle-Bed Reactors”, Chemical Engineering Progress, November 1987, pp. 55-70; Borkink et al., “Influence of Tube and Particle Diameter on Heat Transport in Packed Beds”, AIChE Journal, Vol. 38, No. 5, May 1992, pp. 703-715; Talmor, “Part II. Pulsing Regime Pressure Drop”, AIChE Journal, Vol. 23, No. 6, pp. 874-878; Sai et al., “Pressure Drop in Gas-Liquid Downflow Through Packed Beds”, AIChE Journal, Vol. 33, No. 12, December 1987, pp. 2027-2036; Borio et al., “Cocurrently-Cooled Fixed-Bed Reactors: A Simple Approach to Optimal Cooling Design”, AIChE Journal, Vol. 35, No. 11, pp. 1899-1902, November 1989; Satterfield et al. “Mass Transfer Limitations in a Trickle-Bed Reactor”, AIChE Journal, pp. 226-234, March 1969; Charpentier et al., “Some Liquid Holdup Experimental Data in Trickle-Bed Reactors for Foaming and Nonfoaming Hydrocarbons”, AIChE Journal, Vol. 21, No. 6, pp. 1213-1218, November 1975; and Burghardt et al., Chemical Engineering Science 57 (2002) 4855-4863.
The present inventors have surprisingly discovered a method of reducing the multidimensional problem outlined above to two dimensions.